1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device and a method for fabricating the same to have protrusions, each of the protrusions having a closed loop-shaped cross section, to firmly support column spacers corresponding to the protrusions, thereby preventing the occurrence of display degradation.
2. Discussion of the Related Art
A demand for display devices with various features has been stronger recently. In order to satisfy the consumers' demand, various flat display devices, such as a Liquid Crystal Display (LCD), a Plasma Display Panel (PDP), an Electro Luminescent Display (ELD), and a Vacuum Fluorescent Display (VFD), have been developed, and some of those flat display devices have been widely used in various apparatuses.
Among the flat display devices, the LCD, which has several advantages, such as excellent image quality, a light weight, a thin profile, and a low power consumption, is one of the major substitute for a Cathode Ray Tube (CRT), and is used in various applications, i.e., monitors of TVs set receiving image signals and displaying the image according to the signals, monitors for desktop computers as well as mobile monitors of notebook computers.
In order to use the above LCD as a display device in various fields, it is important to display a high-quality image having a high definition, a high brightness, and a large size, while maintaining low-profile, light-weight and low-power consumption characteristics.
A general liquid crystal display device comprises a first substrate and a second substrate bonded to each other under the condition that a predetermined space is formed therebetween, and a liquid crystal display layer formed between the first and second substrates.
More specifically, in order to define pixel regions, a plurality of gate lines arranged at regular intervals in one direction and a plurality of data lines arranged at regular intervals in another direction, perpendicular to the direction of the gate lines, are formed on the first substrate. Pixel electrodes are respectively formed in the pixel regions, and TFTs for applying data signals of the data lines to the pixel electrodes according to signals applied to the gate lines are respectively formed at intersections between the gate lines and the data lines.
A black matrix layer for preventing light from leaking at portions except for the pixel regions is formed on the second substrate. R, G, and B color filter layers for expressing colors are formed at the portions corresponding to the pixel regions. Common electrodes for forming an image are formed on the color filter layer.
In the above liquid crystal display device, the liquid crystal in the liquid crystal layer formed between the first and second substrates is oriented by an electric field generated between the pixel electrodes and the common electrode, and the amount of the light transmitted by the light crystal layer is adjusted by the degree of the orientation of the liquid crystal layer. Therefore, the liquid crystal display device displays an image.
The above liquid crystal display device is referred to as a Twisted Nematic (TN) mode liquid crystal display device. The TN mode liquid crystal display device has a narrow viewing angle. In order to overcome the above drawback of the TN mode liquid crystal display device, an In-Plane Switching (IPS) mode liquid crystal display device is developed.
In the IPS mode liquid crystal display device, the pixel electrodes and the common electrodes separated from each other by a predetermined distance are formed in parallel in pixel regions of the first substrate to generate a horizontal electric field between the pixel electrodes and the common electrode, and a liquid crystal layer is oriented by the horizontal electric field.
Spacers for maintaining a predetermined interval for the liquid crystal layer between the first and second substrates of the liquid crystal display device are formed on the first or second substrate.
The spacers are divided into ball spacers and column spacers according to shape.
The ball spacers have ball shapes, and are dispersed on the first or second substrate. The ball spacers are comparatively freely movable even after the first and second substrates are bonded to each other, and have a small contact area with the first or second substrate opposite thereto.
On the other hand, the column spacers are formed during an array process performed on the first or second substrate. The column spacers have column shapes with a predetermined height and are fixed to the first or second substrate. Accordingly, the column spacers have a larger contact area with the first or second substrate opposite thereto than that of the ball spacers.
Hereinafter, with reference to the annexed drawings, a conventional liquid crystal display device having column spacers will be described.
FIG. 1 is a sectional view of the liquid crystal display device having column spacers. As shown in FIG. 1, the liquid crystal display device comprises a first substrate 30 and a second substrate 40 facing each other, column spacers 20 formed between the first and second substrates 30 and 40, and a liquid crystal layer (not shown) filled in a space between the first and second substrates 30 and 40.
A plurality of gate lines 31 and a plurality of data lines (not shown), which perpendicularly intersect each other for defining the pixel regions, the TFTs formed at the intersections between the gate lines 31 and the data lines, and the pixel electrodes (not shown) formed in the pixel regions, are disposed on the first substrate 30.
A black matrix layer 41 corresponding to the portions except for the pixel regions, a color filter layer 42 in a stripe shape corresponding to the pixel regions in the lengthwise direction parallel with the data lines, and common electrodes or an overcoat layer 43 formed on the entire surface of the second substrate 40 are disposed on the second substrate 40.
The column spacers 20 correspond to predetermined positions of the upper surface of the gate lines 31. Further, a gate insulating film 36 is formed on the entire surface of the first substrate 30 including the gate lines 31, and a passivation film 37 is formed on the gate insulating film 36.
FIGS. 2A and 2B are plane and sectional views of the liquid crystal display device having column spacers, in which a touch defect occurs. As shown in FIGS. 2A and 2B, when a liquid crystal panel 10 of the above liquid crystal display device having column spacers is touched by hand or using other objects in a predetermined direction, the touched part is stained. This referred to as a touch stain, and generates a touch defect.
It seems that the touch defect is caused by the increase of friction between the column spacers 20 and a first substrate 1 due to a large contact area therebetween, compared to the ball spacers. That is, since the column spacers 20 have a large contact area with the first substrate 1, as shown in FIG. 2B, compared to the ball spacers, it takes a longer time for the first or second substrate 1 or 2, which was shifted by the touch, to return its original state. Accordingly, the touch stain remains until the shifted first or second substrate 1 or 2 has retuned to its original state. The touch stain is well happened when liquid crystal 3 between the first substrate 1 and the second substrate 2 are insufficient.
FIG. 3 is a sectional view of the liquid crystal display device, in which a gravity defect occurs. As shown in FIG. 3, when the liquid crystal display device, is erected in a high-temperature environment, the liquid crystal 3 is thermally expanded and a cell gap is increased more than the height of the column spacers 20. Therefore, the liquid crystal 3 flows down and a gravity defect, in which the lower end of the liquid crystal display device is swollen, occurs.
The above conventional liquid crystal display device having column spacers has several problems as follows.
First, since a contact area between the column spacers and a substrate opposite thereto is large and the friction therebetween is therefore large, when the substrate is shifted by a force, it takes a longer time for the shifted substrate to return to its original state. Accordingly, a touch defect remains until the shifted substrate returns to its original state.
Second, when the liquid crystal display device is erected in a high-temperature environment, the liquid crystal is thermally expanded and a cell gap is increased more than the height of the column spacers. Therefore, the liquid crystal flows down so that the lower end of the liquid crystal display device is swollen, and becomes opaque.